This invention relates to the material and a method for producing an asbestos free calcium silicate material having high heat-resistance which can be machined into any shape or size required.
Heat-resistant materials comprising calcium silicate and having machinability are well known, and are used for manufacturing items such as conduits, troughs, pouring boxes and the like which are used to transport, hold and supply molten metals such as molten aluminum. These heat-resistant board materials are used as they are or are fabricated into various shapes according to their intended uses. For this purpose, calcium silicate board has various advantageous properties such as low density and low thermal conductivity; not being wetted with molten metals; nor staining or in any manner affecting molten metals.
In addition to these properties, other properties such as high mechanical strength, accurate machinability and durability when in contact with hot molten metals, are required. Therefore, ordinary calcium silicate thermal insulation composed of tobermorite or xonotlite cannot be used, for the above purpose. A typical example of a commercially available material conventionally used has been prepared by a special process comprising molding a body made of amosite asbestos and diatomaceous earth with the addition of an inorganic binder; curing the molded body in an atmosphere of steam under high pressure, and heat treating the cured body.
Not only in the above example, but also in other conventional materials, a large amount of asbestos is present. Asbestos has many outstanding advantages; it provides not only high mechanical strength to the final product, but also provides green strength to the molded body prior to its being cured by steam induration and provides stress distribution during curing and heat treating. However, since it is indicated that asbestos fiber is a hazard to human health, it is preferable not to use it.
Under these circumstances, the development of a heat-resistant material free from asbestos is needed. However, an alternative reinforcing fiber which has satisfactory properties comparable to those of asbestos and which can replace asbestos could not be developed heretofore.
With this background, a material and a method for producing a calcium silicate heat-resistant material free from asbestos has been developed, which can be used for conveying molten metals, and to achieve the present invention.
The conventional asbestos free material has the disadvantages that the material becomes hard, brittle and less machinable if the strength is increased. However, we have found that an asbestos free material having both satisfactory strength and machinability without having the above mentioned disadvantages can be obtained by optimally selecting the combination of ingredients and the firing conditions.